ABSTRACT We propose that early endosomes function as a nexus between mitochondria and plasma membrane to regulate a wide variety of cellular processes including receptor-mediated endosomal trafficking, signaling and iron homeostasis. Determining how endosomal alterations on a subcellular level affect specific cancer-related cellular processes, such as cell proliferation, migration and invasiveness is the focus of this research proposal. Here, we will test the hypothesis that alterations in the early endosomal pathway can modify receptor-mediated signaling as well as iron cellular homeostasis in a reciprocal manner to enhance the proliferative and survival properties of cancer cells. We expect that the unravelling of the complex relationship between early endosomes, mitochondria, iron and signaling and cancer progression will provide new tools for cancer therapy and diagnosis. However, current approaches that investigate subcellular cancer cell biology of early endosomal pathway on human breast cancer cells grown in 2D culture are not adequate to fully understand how early endosomes can be re-programmed to support and enhance cancer cell proliferation, survival, migration and/or invasiveness. Since 3D growth has been shown to affect organelle morphology, the analysis of the morphology and function of organelles in 3D tumor systems is the new frontier of cancer cell biology. Here, we will tackle this challenge by studying early endosomes, a complex and dynamic organelle, and their interaction with mitochondria, in a comparative manner across 2D-culture cancer cell lines, 3D breast tumor systems and human tumor frozen tissue sections. In summary, to advance our basic understanding of breast cancer cell biology on a subcellular level, we will investigate the role of the morphology and function of early endosomes and their interaction with mitochondria on the regulation of iron homeostasis and receptor- mediated signaling pathways in 3D breast tumor systems.